A water absorbent resin (SAP/Super Absorbent Polymer) is a water-swelling and water-insoluble polymer gelatinizer, and is used mainly for disposable purpose, i.e., for absorbent articles such as disposable diapers and sanitary napkins, and for an agriculture/horticulture water retention agent, an industrial waterproofing material, and the like. Various kinds of monomers and hydrophilic polymers have been proposed as a raw material for the water absorbent resin. In particular, a polyacrylic acid (salt)-based water absorbent resin in which acrylic acid and/or salt thereof are/is used as a monomer(s) is industrially in widespread use because of its high water absorption performance.
Recently, an absorbent article has been required to contain a high-concentration water absorbent resin, and importance has been placed on liquid permeability of water absorption properties. The reason for this is as follows. A water absorbent resin absorbs water-based liquid to swell. Therefore, a state called a so-called gel blocking is caused in which a water absorbent resin which has come into contact with a water-based liquid swells, and the water-based liquid does not reach a water absorbent resin layer which is distant from the water-based liquid. It is necessary to prevent the water absorbent resin layer which cannot exhibit water absorption performance from occurring.
On the other hand, a mere increase in liquid permeability deteriorates ability to absorb water-based liquid. As a result, a liquid leakage phenomenon is caused in which a water-based liquid which has not been absorbed leaks from an absorbent article. Consequently, a water absorbent resin fails to exhibit a sufficient water absorption performance.
A person skilled in the art has advanced development of various additives so as to improve performance of water absorbent resins. For example, Patent Literature 1 discloses coating a water absorbent resin with polyammonium carbonate obtained by adding carbon dioxide to polyamine so as to prevent adhesiveness from being expressed, the adhesiveness being expressed in a case where the water absorbent resin is merely coated with polyamine, so that the water absorbent resin has an excellent gel bed permeability (GBP). Patent Literature 2 discloses adding, to a water absorbent resin, a suspension containing water-insoluble metal phosphate and polyamine and further containing another hydrophobic polymer, and then heating the mixture at 120° C. or higher, so that the water absorbent resin has excellent saline flow conductivity (SFC), free swell rate (FSR) and fixed height absorption (FHA). Patent Literature 3 discloses a water absorbent resin having excellent saline flow conductivity (SFC) and fixed height absorption (FHA), the water absorbent resin being obtained by adding 10 ppm to 1000 ppm of polyamine and a hydrophobic polymer.
Patent Literature 4 discloses a water absorbent resin having excellent Centrifuge Retention Capacity (CRC) and saline flow conductivity (SFC), the water absorbent resin being obtained by adding a modified cationic polymer containing a primary amino group and/or a secondary amino group.
Patent Literature 5 discloses coating, with water, an aqueous solution, alkanolamine, a polymer and/or a wax, to obtain a water absorbent resin excellent in abrasion resistance. Patent Literature 6 discloses coating at least a surface with a polymer and/or a wax to obtain a water absorbent resin excellent in abrasion resistance. Patent Literature 7 discloses a coating technique with a thermoplastic polymer.
Patent Literature 8 discloses a water absorbent resin which is unlikely to come off from cellulose fiber in an absorbent article, the water absorbent resin being obtained by adding a cationic polymer having a mass average molecular weight of not less than 2000 which cationic polymer can react with a carboxyl group to form an ionic bond.
Patent Literatures 9 and 10 disclose a water absorbent polymer in which a particle of the water absorbent polymer has a crosslinked surface to which powder of water-insoluble phosphate (calcium phosphate in Examples) is fixed with dendrimer (polypropylene imine, polyamide amine or polyester amid). Patent Literatures 9 and 10 further disclose that a resultant water absorbent polymer has high SFC, AAP and CRC.
Patent Literatures 11 and 12 (particularly, for example, Examples 1 through 6 of Patent Literature 11) disclose a water absorbent polymer in which a particle of the water absorbent resin has a crosslinked surface to which powder of polyvalent metal salt (aluminum sulfate in Examples) is fixed with a liquid-permeability adjusting agent (such as polyethylene glycol).
Patent Literatures 13 and 14 disclose a water absorbent polymer in which a particle of the water absorbent polymer has a crosslinked surface to which powder of polyvalent metal salt (powder of aluminum sulfate in Examples) is fixed with an organic polymer bonding agent (polyethylene glycol, such as VORANOL 230-238).
Patent Literature 15 discloses a water absorbent polymer in which a particle of the water absorbent polymer has a crosslinked surface to which powder of a polyvalent metal compound (such as silica or alumina) is fixed with polyethylene glycol.
Patent Literatures 16 and 17 disclose a high-absorbent polymer composition containing (i) a water-insoluble inorganic metal compound (preferably, metal phosphate, titanium borate, aluminum borate, iron borate, magnesium borate, manganese borate or calcium borate) and (ii) a polymer coating (preferably, a polymer coating selected from polyolefin, polyethylene, polyester, polyurethane, polyamide, linear low-density polyethylene, ethylene acrylic acid copolymer, styrene copolymer, ethylene alkyl methacrylate copolymer, polypropylene, maleic acid modified polypropylene, ethylene vinyl acetate copolymer, a blend thereof, and a copolymer thereof).
Patent Literature 18 discloses a high water absorbent polymer having a high liquid permeability, the high water absorbent polymer being coated with water-insoluble inorganic powder and a thermoplastic polymer (preferably, a thermoplastic polymer selected from the group consisting of polyolefin, polyethylene, polyester, polyurethane, linear low-density polyethylene (LLDPE), ethylene acrylic acid copolymer (EAA), styrene copolymer, ethylene alkyl methacrylic acid copolymer (EMA), polypropylene (PP), ethylene vinyl acetate copolymer (EVA), a blend thereof, and a copolymer thereof).
Patent Literature 19 discloses a water absorbent material coated with an elastic membrane forming polymer such as polyether polyurethane.
Patent Literatures 22 through 28 disclose a water absorbent polymer whose liquid permeability (such as SFC and/or GBP) has been improved by using an organic or inorganic polyvalent metal salt (such as aluminum lactate or aluminum sulfate) simultaneously with or separately from surface crosslinking.
Patent Literatures 20 and 21 disclose an oxazolin-based polymer and a polyamide polyamine-epihalohydrin adduct, respectively, as a technique of surface crosslinking a water absorbent resin with a polymer crosslinking agent other than the techniques (described in Patent Literatures 1 through 19) of coating a surface of a water absorbent resin with a polymer compound.
On the other hand, these techniques have problems such as (i) insufficient liquid permeability and water absorbing speed of a resultant water absorbent polymer, (ii) causing coloring of a water absorbent resin, and (iii) causing odor to be emitted. It is therefore desirable to further improve these techniques.
As such, in order to improve liquid permeability (such as SFC), various additives have been proposed. Though these additives (particularly, addition of a polymer) improve the liquid permeability, these additives generally deteriorate fixed height absorption (FHA) and water absorbing speed (such as FSR), particularly the FHA. Moreover, a relatively large amount of additive (particularly, a polymer) (for example, in % order) is required to bring about an effect. Accordingly, these additives are disadvantageous in terms of cost and absorption capacity. These conventional techniques further have difficulty attaining both the liquid permeability (such as SFC) and the FHA (and the water absorbing speed).